The problems associated with the use of high carbon content coal ash as a constituent material for contemporary cementitious products are well known. Scores of articles have been published regarding coal ash adsorption of air-entraining admixtures leading to inconsistent air content amongst cement pours and ultimately resulting in premature concrete failure.
Researchers have identified three distinctly different carbon types in coal ash samples taken from different ashes all considered to be “high carbon content coal ash”. These microscopically identifiable carbon types are inertinite, isotropic coke and anisotropic coke. The three carbon types have distinctly different chemical and physical properties leading to different adsorption capacities for air entraining admixtures when used in cementitious systems. Variability in adsorption potential for high carbon coal ash often results in significant variability in concrete performance with regard to both slump characteristics and freeze-thaw durability. Such variation in pertinent concrete performance properties has resulted in high carbon content coal ash being excluded for use in many cementitious applications. An noteworthy exclusion criterion is that promulgated by a state agency regulating materials used in the construction of various highways, in which case, for example, the loss of ignition (LOI) for fly ash to be used in concrete is not to exceed three percent.
It is important to note the carbon materials inertinite, isotropic coke and anisotropic coke are not the only carbon components comprising coal ash with potentially detrimental consequences for use in contemporary cementitious applications. Various fly ash waste streams are also known to contain small percentages of activated carbon. Activated carbon is often injected into flue gas streams to prevent nuisance elements from being discharged to the atmosphere—with an example of a nuisance element being mercury.
In past regulatory environments, markets existed for fly ashes containing high contents of carbon—for example, coal ashes with LOI greater than three percent. During such times, one routine outlet for high carbon content fly ash was its use as feedstock to cement kilns. Various fly ashes are often high in silica-and alumina-containing compounds, making these materials valuable contributors to cement production from a technologic point of view. Although fly ash serves as a suitable raw material for cement production, new industrial emissions standards may endanger such a promising outlet for fly ash. The underlying reason is that fly ashes also contain nuisance materials such as arsenic, mercury, lead, cadmium and zinc, to name a few. When introduced to the high operating temperatures of a cement kiln, these materials have a tendency to transition into their respective vapor phases, thus becoming gaseous emissions. New emissions standards may force cement producers to seek raw materials that will not be significant contributors to unwanted gaseous pollutants. Fly ashes once destined for use in cement kilns will now likely end up in landfills. It is becoming apparent that the amended Clean Air Act in the United States and similar regulatory regimes in other jurisdictions create quite the conundrum for both the coal-fired utilities industry and the cement industry. New regulations are cleaning up the air, but these regulations are continuing to pressure industry with regard to finding outlets for waste streams.
The invention herein described is useful for diminishing negative effects associated with problematic coal ash when used either in combination with other hydraulic binders or as a hydraulic binder. Typically, problematic coal ash falls outside specifications provided by ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. The invention, according to which, inter alia, coal ash and latex polymer are mixed, provides either a partial or full coating for coal ash particles which acts as a diffusion barrier, diminishing the adsorption capacity of the coal ash, thus rendering it usable in cementitious materials. Mixing problematic coal ash with any form of latex polymer, whether liquid dispersion or dispersible polymer powder, enables use of problematic coal ash in cementitious materials, providing a viable “green” alternative when compared to traditional landfilling of such problematic coal ash.
Problematic coal ash may typically result from lower process temperatures employed in coal burning processes. Such low temperatures cause coal ash morphology to differ from the morphology characteristic of coal ash produced during past regulatory regimes. Problematic coal ash resulting from lower process temperatures is likely to be more “platy” in shape, whereas coal ash which has been burned at high temperatures may appear completely fused and round. Platy particle morphology often results in increased water demand for cementitious materials, as the platy particles do not flow as well as round particles. Increased water demand often creates a number of troubles for cementitious materials, with two notable problems being increased pore diameter and increased rates of harmful ion ingress.
There is accordingly a long-felt need for a process for diminishing the adsorption capacity of coal ash.